1. Field of the Invention
The invention relates to a quartz plate used in a quartz resonator or a quartz device such as a quartz oscillator, and a quartz resonator in which the quartz plate is used.
2. Description of the Related Art
An AT-cut quartz resonator is an electronic device used for various purposes, for example, it is often used in communication tools. The AT-cut quartz resonator, when loaded in vehicles, is used as a timing device which creates a reference frequency of ECU (electronic control unit). As substrates of the ECU are dimensionally smaller in recent years, an installation space of the quartz resonator is increasingly narrowed.
The quartz resonator should be thin, outside dimensions of which conventionally required are, for example, about 3.2 mm in length×2.5 in width. A low frequency, 8.000 MHz, for example is used as the ECU reference frequency.
An AT-cut quartz plate has an oscillation frequency in reverse proportion to its own thickness dimension, meaning that the quartz plate has a larger thickness as the frequency is lower. Therefore, a quartz plate for such a low frequency as 8.000 MHz has a larger thickness than a quartz plate for a higher frequency.
Usually, the quartz resonator has a structure where end portions of the quartz plate are bonded with, for example, an adhesive in a container to be supported therein. Because of larger thicknesses of the quartz plate in the band of low frequencies, the end portions of the quartz plate are conventionally chamfered (generally called beveling work) to attenuate a vibration energy generated in an area of the container where the quartz plate is supported and to obtain favorable series resistance values by enclosing the vibration energy under driving electrodes formed in center portions of front and back surfaces of the quartz plate.
To subject the quartz plate to the beveling work, an abrasive and the quartz plate are sealed in a working chamber. A plurality of the working chambers thus containing the abrasive and the quartz plate are set in a high-speed rotary tank to leverage a centrifugal force generated when the working chambers thus set are rotated at high speeds.
A shape of the beveled quartz plate (bevel shape) is reflected on resonance waveforms. More specifically, frequencies of primary vibration (resonance) and sub-vibration (unwanted vibration) of the quartz plate are variable depending on the progress of the beveling work. Therefore, the bevel shape can be suitably controlled by confirming positions (frequencies) of the primary and sub-vibrations using, for example, a network analyzer.
The patent references, JP Patent No. 2075046, JP Patent No. 4623321, and the specification of Unexamined Japanese Patent Applications Laid-Open No. 2007-335941 disclose the quartz plates controlled based on frequency differences between the primary vibration and sub-vibration and the beveling work management method.
The frequency of the primary vibration of the AT-cut quartz plate disclosed in the specification is 26 MHz, therefore, the quartz plate according to the invention is thinner than the 8-MHz quartz plate. On the other hand, a quartz plate, wherein the primary vibration frequency is 8 MHz, is as thick as about 208 μm which is three times larger than the thickness of the 26-MHz quartz plate.
With such an increasing thickness of the quartz plate, it becomes more important to attenuate the vibration energy by suitably beveling the quartz plate. Therefore, characteristics of the bevel shape have a larger impact on the quartz plate wherein the frequency of the primary vibration is 8 MHz (low frequency) than the quartz plate wherein the frequency of the primary vibration is 26 MHz.
For instance, the series resistance value is 500Ω or larger in a quartz plate that can be housed in a surface mounting quartz resonator wherein outer dimensions in planar view are 3.2 mm in length×2.5 mm in width and the primary vibration frequency is 8 MHz. Thus, it was difficult to find a bevel shape suitable for achieving desirable series resistance values, and no quantitative index for reliably reproducing such a suitable bevel shape was available.